Perfluoroalkyl group-containing carboxylic acids (salts) are widely used in the emulsion polymerization of fluorine-containing monomers as fluoro-based surfactants that act as emulsifiers. In particular, perfluorooctanoic acid C7F15COOH or a salt thereof (hereinafter abbreviated as [PFOA]) is known as a surfactant having excellent monomer-emulsifying properties and latex stability.
However, perfluorinated chemical substances are difficult to degrade in the natural environment. Further, it has recently been revealed that C8 perfluorinated compounds, typified by PFOA, are extremely remaining in the human body. In addition, due to the high affinity of PFOA to polymers, after agglutination of polymer latex obtained by emulsion polymerization, a large amount of PFOA adheres or remains in the fluorine-containing polymer. There is a strong demand for reducing the amount of adhering or remaining PFOA.
As a means for imparting environmental degradability to fluorine-containing emulsifiers, it is possible to provide a hydrogenated portion in the perfluorinated hydrophobic group of the surfactant compound. Moreover, as a means for lowering the environmental remaining of decomposition products, it is considered desirable that the hydrophobic group has a perfluorinated continuous carbon chain with less than 8 carbon atoms (e.g., CnF2n+1CpH2pCqF2q— (m, q: 1 to 7; p: 1 or more)).
Patent Document 1 describes the use, as a dispersing agent, of a mixture of a compound represented by the general formula:F(CF2CF2)nCH2CH2SO3M (M: monovalent cation)wherein n is a cardinal number of 2 to 8 and the average value of n is 2 to 6, and a compound of the above formula wherein n is a cardinal number of 2 to 6. However, when this perfluoroalkyl ethanesulfonic acid (salt) is used as a fluorine-containing surfactant, chain transfer occurs during the polymerization reaction. Consequently, the resulting polymer inevitably has a lower molecular weight.
As a hydrogen-containing, fluoro-based surfactant that is less likely to undergo chain transfer during the polymerization reaction, Patent Document 2 proposes the use of a compound represented by the general formula:Rf(CH2)mR′fCOOM                Rf: C3-C8 perfluoroalkyl group or perfluoroalkoxyl group        R′f: C1-C4 perfluoroalkylene group        M: NH4, Li, Na, K, or H        m: 1 to 3.However, when used as an emulsifier for the emulsion polymerization of fluorine-based monomers, this compound is inferior to PFOA in terms of monomer-emulsifying properties and latex stability. Further, in homopolymerization or copolymerization of vinylidene fluoride, the low micelle solubility of the vinylidene fluoride as a monomer significantly lowers the polymerization rate. In addition, the obtained polymer latex has poor stability, and precipitates may be produced during the polymerization reaction.        
Moreover, the hydrogen-containing, fluoro-based surfactant Rf(CH2)mRf′COOM described in Patent Document 2 is obtained by alkali hydrolysis of the corresponding carboxylic ester to form a free carboxylic acid, followed by reaction with a base to form a carboxylic acid salt. Since the alkali hydrolysis of the carboxylic ester is carried out in the presence of excess amount of alkali, a side reaction occurs to produce about several percent of product in which the —CH2CF2— group in the molecule is converted to a —CH═CF— group by an HF-elimination reaction.
Furthermore, there have been attempts to use fluorine-containing polyether carboxylic acids (salts) as emulsifiers (see Patent Document 3); however, many of this type of compounds have low solubility in aqueous media and are extremely difficult to handle. Further, washing after coagulation is not easy. Among polyether-based carboxylic acid (salt) emulsifiers, this tendency is particularly remarkable in those having side chains or a long chain.
It is also described that polyether carboxylic acids (salts) are easy to bubble, the latex obtained by polymerization is not easy to handle, and the amount of aggregate in the latex is greater than that of PFOA (see Patent Document 3).
In addition, polyfluoroalkylphosphonic acid esters are widely used as starting materials for the synthesis of mold-releasing agents. Compounds having a C8-C12 perfluoroalkyl group are most likely to develop mold release performance when used as mold-releasing agents. In particular, perfluorooctyl group-containing compounds of the formula:CF3(CF2)7CH2CH2P(O)(OC2H5)2 are preferably used for this kind of application (see Patent Documents 4 to 7).
As described above, it is reported that compounds having a C8-C12 perfluoroalkyl group are biologically degraded in the environment and converted to compounds having relatively high bioaccumulation and environmental concentration, causing concerns for exposure during treatment processes, and for release or diffusion from waste, treated substrates, etc., into the environment. Moreover, compounds having a perfluoroalkyl group containing 14 or more carbon atoms are very difficult to handle because of their physical and chemical properties. Hence, such compounds are rarely used in practice.
Furthermore, as for telomer compounds having a perfluoroalkyl group containing 8 or more carbon atoms, generation and mixing of perfluorooctanoic acids with a high bioaccumulation potential is unavoidable during the production of these compounds. For these reasons, companies that produce such telomer compounds have retreated from the production of the compounds or promoted the use of alternative compounds having a perfluoroalkyl group containing 6 or less carbon atoms.
However, compounds having a perfluoroalkyl group containing 6 or less carbon atoms cause a significant decrease in orientation on the surface of a treated substrate, and the melting point, glass transition point (Tg), etc., of the compounds are markedly lower than those of C8 compounds. Accordingly, the compounds are highly influenced by their environmental conditions, such as temperature, humidity, stress, and contact with organic solvents. Consequently, the desired performance cannot be sufficiently achieved, and durability and other properties are affected.
Fluorine-containing polymer resins, which are fluorine-containing polymers, are widely used for coating materials because of their excellent antifouling properties and weather resistance. Further, fluorine-containing polymer elastomers have excellent heat resistance, oil resistance, chemical resistance, etc. Vulcanization molded products thereof are widely used as various sealing materials, such as oil seal, O rings, packing, and gaskets.
Aqueous dispersions of fluorine-containing polymers used for this type of application are subjected to mechanical processes (e.g., filtration through a metal sieve) when used; therefore, they are required to have sufficient stability therefor.